You'd have to work really hard to vet, and then continue to re-vet, your sellers in order to keep the crap out. There's just too much potential for profit in abusing the system, managing to sell your $1.50 sweatshop knit hats for $40.

You'd want to start off with carefully-selected invites to specific sellers, and then maybe do a referral-based system to bring on additional sellers. Video calls and photo ID for verification. Reporting systems if buyers get mass-produced junk.

Sounds like it would take quite a bit of human effort and care so I can't imagine it getting funded :)